1. Field of Invention
This invention relates generally to wireless communication systems. Specifically, the invention relates to a cancellation system for use with wireless communication systems.
2. Related Art
Current communication systems usually support multi-mode wireless communication standards where the mobile device may operate and exist with a small form factor. Specifically, with the wide variety of communications standards that modern wireless systems are required to comply with, interference between the various wireless systems or sub-systems can be a true design challenge. Unlike the case of a single isolated wireless system, where possible interference between the transmitter and receiver can be handled by the use of different frequencies or different time slots, the co-existence of more than one system in close proximity can make the interference problem much more severe. In this case where more than one transmitter operate at the same frequency and same time slot, a receiver will detect undesired interference, which could overwhelm the desired signal and cause significant degradation in the quality of the received signal.
Moreover, the distance between the various antenna becomes a critical issue in such a complex communication environment where a number of transmitter and receiver systems are required to co-exist. In some cases, where the antenna are located in the Fresnel near-field region, that may result in constructive and destructive signal combining that may produce a large tone that would overpower the smaller reflected signal from the target. For example, assuming the simple case of a large transmitted tone to co-exist with the received signal at the same (or close to) frequency and time slot of the received signal. In practice, the desired received signal may be as low as 90 dB to 130 dB below the large tone. This result would effectively put the reflected signal level below the noise floor of most spectrum analyzers due to compression as well as the transmitter noise spectrum, which typically at best have approximately 60 dB of dynamic range (where dynamic range is generally defined as the difference between the highest and lowest power signals that the spectrum analyzer can simultaneously measure).
Typically, the magnitude of this large tone may be decreased by spacing the adjacent antennas farther apart; however, in many applications, space is premium such that the antenna cannot be spaced farther apart. This issue with simultaneous transmit and receiving problem is the same fundamental limit that forces majority of communication systems to operate as a “half-duplex” system, i.e. as only a transmitter or receiver at any point in time. Unless the frequency bands of the transmit signal and receiver signal are separated with enough margin to allow for a duplexer filter to be used, a physical realization of a “full-duplex” transceiver system is very difficult to realize. That is mainly due to the difficulty of receiving weak signals into a receiver in the existence of a strong interferer coming from the strong transmitted signal from the same system or another adjacent transmitter system that can be in proximity.
As an example of these problems in FIG. 1, a block diagram of an example of an implementation of a wireless transceiver 100 for use with a communication system is shown. The transceiver system 100 may include a first antenna 102, a second antenna 104, a transmitter 106, a receiver 108, a first frequency source 110, and a second frequency source 112. The transmitter 106 may be in signal communication with both the first antenna 102 and first frequency source 110 via signal paths 114 and 116, respectively. Similarly, the receiver 108 may be in signal communication with both the second antenna 104 and second frequency source 112 via signal paths 118 and 120, respectively. Additionally, the first antenna 102 may be in signal communication with the second antenna 104 via signal paths 122.
In an example of operation, the transmitter 106 receives a frequency reference signal 124 from the first frequency source 110 via signal path 116. The transmitter 106 then transmits a transmit signal 126 through the first antenna 102 which becomes signal 128. The signal 130 is the transmitter leakage that directly couples to the second antenna 104 via signal path 122.
The second antenna 104 receives the first portion 130 of the transmit signal 126 as well as the received signal 132 and passes the combined received signal 134 to the receiver 108, via signal path 118, which produces an receiver output signal 136.
In FIG. 2, an example plot of the amplitude 200 versus frequency 202 of the receiver output signal 200 produced by the transceiver system 100 of FIG. 1 is shown. The receiver output signal 204 may include transmitter leakage 206 at frequency F0 208 (which would be the local oscillator frequency of the first frequency source 110) and a secondary tone at frequency F1.
The transmitter leakage 206 is caused by the direct coupling from the transmit antenna to the receive antenna as shown in FIG. 1. The difference in amplitude intensity between the transmit leakage 206 and the received signal 210 is shown as A 214. As an example, the difference between the transmitter leakage 206 and the received signal 210 may be as much as 90 to 130 dB which effectively places the received signals 210 below the noise floor coming from the transmitted signal. Additionally, because of jitter effects caused by the characteristics of the frequency source and the relatively small frequency and time difference between the transmitter leakage 206 and the received signal 210, the transmitter leakage 206 may have a skirt 216 that effectively covers the received signal 210 and will, hence, make it impossible to recover the desired signal. As such, there is a need for a methodology to extend the dynamic range of transceiver system that overcomes the above mentioned problems.